Dr. P. Suganya
Department of Biotechnology
Sri Kaliswari College (Autonomous)
What is DNA?
“DNA is a group of molecules that is responsible for
carrying and transmitting the hereditary materials or the
genetic instructions from parents to offsprings.”
• DNA stands for Deoxyribonucleic Acid which is a molecule
that contains the instructions an organism needs to develop,
live and reproduce.
• These instructions are found inside every cell and are
passed down from parents to their children.
• It is a nucleic acid and is one of the four major types of
macromolecules that are known to be essential for all forms
• DNA is found in the nucleus, with a small amount of DNA
also present in mitochondria in the eukaryotes.
• In 1953, James Watson and Francis Crick discovered
the structure of DNA.
• The works of Rosalind Franklin lead to Watson and Crick’s
discovery. Franklin first had pointed out that the DNA is made up
of two spirals.
• The structure of DNA is a double helix structure because it looks
like a twisted ladder.
• The sides of the ladder are made of alternating sugar
(deoxyribose) and phosphate molecules while the steps of the
ladder are made up of a pair of nitrogen bases.
• There are 4 types of nitrogen bases Adenine (A) Thymine (T)
Guanine (G) Cytosine (C) DNA Pairing. The nitrogen bases have
a specific pairing pattern.
• This pairing pattern occurs because the amount of adenine
equals the amount of thymine; the amount of guanine equals
the amount of cytosine. The pairs are held together by
• DNA is a double-stranded helix.
• That is each DNA molecule is comprised of two biopolymer
strands coiling around each other to form a double helix
• These two DNA strands are called polynucleotides, as they
are made of simpler monomer units called nucleotides.
• Each strand has a 5′end (with a phosphate group) and a
3′end (with a hydroxyl group).
• The strands are antiparallel, meaning that one strand runs
in a 5′to 3′direction, while the other strand runs in a 3′ to 5′
• The two strands are held together by hydrogen bonds and
are complimentary to each other.
• Basically, the DNA is composed of deoxyribonucleotides.
• The deoxyribonucleotides are linked together by 3′ –
• The nitrogenous bases that compose the deoxyribonucleotides
include adenine, cytosine, thymine, and guanine.
• The complimentary of the strands are due to the nature of the
• The base adenine always interacts with a thymine (A-T) on the
opposite strand via two hydrogen bonds and cytosine always
interacts with guanine (C-G) via three hydrogen bonds on the
• The shape of the helix is stabilized by hydrogen bonding and
hydrophobic interactions between bases.
• The diameter of double helix is 2nm and the double helical
structure repeats at an interval of 3.4nm which corresponds
to ten base pairs.
Major and Minor Grooves of the DNA
• As a result of the double helical nature of DNA, the molecule has
two asymmetric grooves. One groove is smaller than the other.
• This asymmetry is a result of the geometrical configuration of the
bonds between the phosphate, sugar, and base groups that forces
the base groups to attach at 120 degree angles instead of 180
• The larger groove is called the major groove, occurs when the
backbones are far apart; while the smaller one is called the minor
groove, occurs when they are close together.
• Since the major and minor grooves expose the edges of the bases,
the grooves can be used to tell the base sequence of a
specific DNA molecule.
• The possibility for such recognition is critical, since proteins must
be able to recognize specific DNA sequences on which to bind in
order for the proper functions of the body and cell to be carried
Properties of DNA
• DNA helices can be right handed or left handed. But the B –
conformation of DNA having the right handed helices is the
• On heating the two strands of DNA separate from each other
and on cooling these again hybridize.
• The temperature at which the two strands separate
completely is known as melting temperature (Tm). Melting
temperature is specific for each specific sequence.
• The B sample of DNA having higher melting point must have
more C-G content because C-G pair has 3 hydrogen bonds.
• The sequence of bases along the DNA molecule encodes for
the sequence of amino acids in every protein in all
Types of DNA
• Eukaryotic organisms such as animals, plants and fungi,
store the majority of their DNA inside the cell nucleus and
some of their DNA in organelles such as mitochondria.
• Based on the location DNA may be
• Nuclear DNA
• Located within the nucleus of eukaryote cells.
• Usually has two copies per cell.
• The structure of nuclear DNA chromosomes is linear with
open ends and includes 46 chromosomes containing 3
• Nuclear DNA is diploid, ordinarily inheriting the DNA from
two parents. The mutation rate for nuclear DNA is less than
• Mitochondrial DNA is located in the mitochondria.
• Contains 100-1,000 copies per cell.
• Mitochondrial DNA chromosomes usually have closed,
circular structures, and contain for example 16,569
nucleotides in human.
• Mitochondrial DNA is haploid, coming only from the mother.
• The mutation rate for mitochondrial DNA is generally higher
than nuclear DNA.
• Most of the DNA is in the classic Watson-Crick model simply
called as B-DNA or B-form DNA.
• In certain condition, different forms of DNAs are found to be
appeared like A-DNA,Z-DNA,C- DNA,D-DNA,E-DNA.
• This deviation in forms are based on their structural diversity.
• Most common, originally deduced from X-ray diffraction of
sodium salt of DNA fibres at 92% relative humidity.
• Originally identified by X-ray diffraction of analysis of DNA fibres
at 75% relative humidity.
• Left handed double helical structure winds to the left in a zig- zag
• Formed at 66% relative humidity and in presence of Li+ and
• Rare variant with 8 base pairs per helical turn, form in
structure devoid of guanine
• E- DNA
• Extended or eccentric DNA.
Functions of DNA
• DNA has a crucial role as genetic material in most living
organisms. It carries genetic information from cell to cell
and from generation to generation.
• Thus its major functions include:
• Storing genetic information
• Directing protein synthesis
• Determining genetic coding
• Directly responsible for metabolic activities, evolution,
heredity, and differentiation.
• It is a stable molecule and holds more complex information
for longer periods of time.
• Bailey, W. R., Scott, E. G., Finegold, S. M., & Baron, E. J.
(1986). Bailey and Scott’s Diagnostic microbiology. St. Louis:
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